Chapter 3 – Polymer Lecture Review

Introduction to Polymers

• Polymer = chemical compound whose molecules are bonded in long, repeating chains.
• Occur naturally (rubber, shellac, cellulose) and can be synthesized (polyethylene, polystyrene, etc.).
• Natural examples:
– Rubber: excellent elasticity—molecular chains created by nature; used for millennia.
– Shellac: insect‐secreted resin, primer/sealant/varnish.
– Cellulose: most abundant natural polymer; cell walls; paper, textiles, cellophane.
• Synthetic examples:
– Polyethylene (PE): most common plastic; bags, containers.
– Polystyrene (PS): packing peanuts, disposable cups.

General Characteristics of Polymers

1. Chemical resistance
   • Many household chemicals stored in polymer packages.
   • Some solvents dissolve specific plastics; others remain inert.

2. Thermal & electrical insulation
   • Appliance casings, cords, handles; polypropylene underwear, acrylic fiberfill, foam insulation, microwave cookware.

3. Low density + strength/weight advantages
   • Applications from nylon pantyhose to Kevlar vests; lighter than stone, concrete, metals.

4. Processability
   • Extrusion (fibers, pipes, films), injection molding (intricate parts), blow molding (bottles), foaming (Styrofoam™).
   • Stretchability (soft‐drink PET bottles), flexible elastomers.

5. Tunable properties & color
   • Additives can replicate cotton, silk, marble, metals; create clear sheets, flexible films.

6. Feedstock sources
   • Mostly petroleum/natural gas/coal; renewables emerging (polylactic acid from corn, cellulose acetate, etc.).

7. Unique applications with no alternative
   • Clear waterproof films, medical PVC tubing/blood bags, heparin‐impregnated catheters, anti‐thrombogenic surfaces.

Advantages (Selected Functional Traits)

• Reflectivity (specialty films)
• Impact resistance (luggage, bumpers)
• Brittleness when desired (certain PS grades)
• Translucence (arts & crafts clays)
• Ductility (stretch without fracture)
• Elasticity (natural/synthetic rubbers → tires, seals)

Polymerization Fundamentals

• Synthetic polymers manufactured by joining many monomers → long chains via covalent bonds.
• Two principal mechanisms:
– Step-growth polymerization: monomers bond directly; molecular weight rises slowly.
– Chain-growth polymerization: chain grows by adding one monomer at a time.
• Chain structure governs properties:
– Tightly twisted/strong bonds → strong, tough polymer.
– Stretchy segments → flexible polymer.
• Glass-transition reduction via plasticizers: $T_1$ lowered below room temperature to yield ductility.

2.0 Types of Polymer – Overview

2.1 Classification by Mode of Polymerisation

• (Not elaborated but refers to step-growth vs chain-growth.)

2.2 Classification by Molecular Forces

• Elastomers
• Fibers
• Liquid Resins
• Plastics
– Thermoplastics
– Thermosetting plastics

Elastomers

• Rubber-like solids; weakest intermolecular forces; stretch & retract via cross-links.
• Examples & uses:
– Natural rubber: gaskets, shoe heels.
– Polyurethanes: Lycra (elastic apparel), foams, wheels.
– Polybutadiene: high-wear tires.
– Neoprene: wetsuits, wire insulation, belts.
– Silicones: pacifiers, prostheses, lubricants, molds; high thermal/chemical resistance.

Fibers

• Strong intermolecular forces, low elasticity, high tensile strength, thread-like, easily woven.
• Example: Nylon (carpets, apparel); sharp/high melting point.

Liquid Resins

• Adhesives, potting compounds, sealants delivered in liquid state.
• Examples: epoxy adhesives, polysulfide sealants.

Plastics (General Category)

• Hard/tough utility articles via heat & pressure.
• Common: Polystyrene, PVC, PMMA.
• Sub-classes:
– Thermoplastics
– Thermosets

Thermoplastics

• Long chain molecules; soften on heating, harden on cooling—reversible, allowing recycling & remolding.
• Pros: highly recyclable, good aesthetics, impact-resistant, chemical-resistant, eco-friendly production, reshaping capability.
• Cons: generally costlier, may melt under heat.
• Performance varies by resin: bags (low-stress) to mechanical parts (high-stress).

Thermosetting Plastics (Thermosets)

• Cross-link during curing → irreversible network.
• Pros: withstand higher temperatures, superior dimensional stability, thick/thin wall capability, cost-effective.
• Cons: non-recyclable, difficult surface finishing, cannot be reshaped.

Plastics Safety: Resin Identification Codes (RIC)

SAFER vs TOXIC list (per TheSoftLanding.com):
• Safer: #1 PET (single-use bottles), #2 HDPE (milk jugs, detergent), #4 LDPE (bags, squeeze bottles), #5 PP (sippy cups, straws)
• Toxic/Use Caution: #3 PVC (vinyl toys, medical tubing), #6 PS (foam cups, take-out containers), #7 OTHER (polycarbonate & unknown blends; beware BPA)

Health/Environmental Hazards by Code

• PET (#1): may leach antimony trioxide, phthalates → cancer, endocrine issues.
• HDPE (#2): can leach nonylphenol under sunlight → endocrine disruption.
• PVC (#3): contains BPA, lead, phthalates, mercury, dioxins, cadmium → carcinogenic, endocrine, respiratory issues.
• LDPE (#4): potential nonylphenol leaching under sunlight.
• PP (#5): relatively safe; possible additive leaching; occupational asthma.
• PS (#6): leaches styrene → carcinogen, neurotoxin; animal studies show organ/genetic harm.
• OTHER/PC (#7): leaches BPA → infertility, cancers, metabolic disorders.

Polyethylene vs Polypropylene (Comparison)

• Monomer units: PE from ethylene, PP from propylene.
• Rigidity: PP > PE (PP rigid; PE flexible).
• Transparency: PE translucent; PP normally opaque but bleach-able.
• Static charge: PP carries higher static.
• Melting point: $T<em>{m,PP} > T</em>{m,PE}$ (PP higher heat tolerance).

3.0 Polymer Additives

Purpose: enhance mechanical properties, processing ease, durability; mitigate heat/UV degradation.
• Fillers: improve tensile strength, abrasion resistance; lower cost (carbon black, silica gel, wood flour, glass, limestone, talc).
• Plasticizers: lower glass transition $T_g$; impart ductility (common in PVC).
• Stabilizers: antioxidants, UV absorbers.
• Lubricants: processing aid; polymer "slides" through dies (e.g., sodium stearate).
• Colorants: dyes & pigments.
• Flame Retardants: halogenated or boron compounds.

4.0 Polymer Manufacturing Processes

General Notes

• Operations parallel metal forming: molding, cutting, machining, joining.
• Materials shipped as pellets/powders; melted just before shaping.
• Process parameters strongly affect final properties; coloring, finish, precision often obtained in-process; large complex shapes can be molded as single units.
• Fabrication methods include casting, blow molding, compression molding, transfer molding, cold molding, injection molding, reaction injection molding, extrusion, thermoforming, rotational molding, foam molding.

1. Casting

• Simplest: no fillers, low/atmospheric pressure.
• Castable thermoplastics: acrylics, nylons, urethanes, PVC plastisols.
• Castable thermosets: phenolics, polyesters, epoxies, silicones, urethanes.
• Produces lustrous, transparent/translucent parts.
• Process (thermoset example): steel mandrel → dip in molten lead → lead shell mold → resin fill → cure $65!^{\circ}\text{C} - 95!^{\circ}\text{C}$ → demold.

2. Blow Molding

• Inflates hot preform (parison) within closed mold via compressed air.
• Produces hollow parts (bottles, containers).
• Main variants: extrusion blow, injection blow, injection-stretch blow.
• Steps: extrude/inject parison → capture in split mold → air pressure expansion → cool → eject.

3. Compression Molding (Hot-Compression)

• Thermoset granules/preforms placed in heated mold; heat softens, pressure shapes.
• Suited for high-volume, high-strength, fiberglass-reinforced components.
• Sequence: load charge → close mold under hydraulic press → cure → eject via pins.

4. Transfer Molding

• Metered charge preheated in pot; plunger forces material through runner into closed mold.
• Higher dimensional accuracy than compression molding; less environmental exposure.
• Common for thermosets; includes sprue, gates, ejector pins.

5. Cold Molding

• Uncured thermoset pressed cold to shape, ejected, then oven-cured.
• Pros: faster, economical.
• Cons: poorer surface finish, dimensional precision; heavy parts, limited colors, dull finish.

6. Injection Molding

• Widely used for thermoplastics & thermosets; analogous to die-casting.
• Process: pellets → hopper → heated barrel → reciprocating screw mixes/melts → inject into mold cavity → cool/solidify → open mold → eject.
• Machine units: injection (screw, barrel, nozzle, heaters) & clamping (platens, tie rods, hydraulic cylinder).
• Defects: burn marks, voids, incomplete fill, weld-line weakness; mold issues (abrasion, corrosion) due to "diesel effect" (combustion gases).
• Operational repercussions: clogged vents, longer cycle, higher energy.

7. Extrusion

• High-volume continuous profile production (pipes, films, window frames).
• Feed → rotating screw → melting by shear + heaters → through breaker plate → die → continuous extrudate.
• Screw sections: feed, compression, metering.

8. Thermoforming (incl. Vacuum Forming)

• Heat plastic sheet to pliable state → drape over/into mold → vacuum + pressure pull sheet to contours → cool → trim.
• Thin-gauge: disposable cups, lids, blisters.
• Thick-gauge: vehicle panels, refrigerator liners.

9. Rotational Molding (Rotomolding)

• Hollow mold charged with powder/liquid resin → bi-axial rotation in oven → resin coats interior → cool while rotating.
• Pros: good surface & dimensional tolerance, machining eliminated.
• Cons: high cost, size/weight limitations, demolding challenges.

10. Foam Molding

• Foaming agent mixed with resin; gas release on heating expands material $\times 2\text{–}50$ original volume.
• Densities: $32 - 640\;\text{g/L}$; open-cell (permeable) vs closed-cell (gas-tight) foams.

Ethical, Environmental & Practical Considerations

• Recycling critical: thermoplastics can be reprocessed; thermosets cannot.
• Additive leaching raises health concerns (BPA, phthalates, heavy metals).
• Renewable feedstocks (PLA, cellulose) reduce fossil dependence.
• Medical reliance on polymers demands biocompatibility, sterilizability, and safe disposal.

Quick Reference: Key Equations & Symbols

• Glass transition reduction: $T<em>g \downarrow \quad\text{via plasticizer}$ • Melting points: $T</em>{m,PE} < T<em>{m,PP}$ • Density of foam products: $\rho</em>{foam}=\frac{m}{V} \to 32\text{–}640\;\text{g/L}$ (after expansion)